A piece of film from some bad negatives taped to your webcam lens is all you need for daytime, that's not too difficult(ok, if you're of the digital age, ask your mom/grandma). Turning of the lights after dark isn't too hard either, and I've used everything from a lighter, to my mouse, to my cellphone as the targeting light source.

I've done some testing using specific color values, but the issue is with reflected light. Even without magenta existing in the real world, it does exist in reflected light in a webcam since the true colors are resolved to a limited subset of the spectrum (256,256,256). A single point of light the color of magenta away from your targeting dot will make the colorBoundsRect paramaters worthless.